The present disclosure is generally related to a method of preparing cyclic monomers for ring-opening polymerizations, in particular cyclic carbonates, cyclic carbamates, cyclic ureas, cyclic thiocarbonates, cyclic thiocarbamates, and cyclic dithiocarbonates.
Technological advances continue to present many complex environmental issues. As a consequence, pollution prevention and waste management constitute two significant challenges of the 21st century. “Green” chemistry is a concept that is being embraced around the world to insure continued economic and environmental prosperity. The interest in the U.S. began with the passage of the Pollution Prevention Act of 1990, the first law focused on the source rather than the remediation of the pollutants, which prompted the U.S. Environmental Protection Agency (EPA) to establish its green chemistry program in 1991. Since then, modern synthetic methodologies are encouraged to preserve performance while minimizing toxicity, use renewable feedstocks, and use catalytic and/or recyclable reagents to minimize waste. Green chemistry is the design and development of chemical products/processes that reduce or eliminate the use of substances harmful to our health or environment. What makes green chemistry such a powerful concept is that it starts at the molecular level and ultimately generates environmentally benign materials or material processes.
Phosgene is produced on a 10,000 ton scale per year for the formation of isocyanates (for making polycarbamates), polycarbonates (e.g., bisphenol A polycarbonate), and the formation of acid chlorides. Although phosgene is widely used, it is expensive and toxic. Phosgene was used in World War I as a chemical weapon and has been involved in tragic industrial accidents. Phosgene can be detected at 0.4 ppm which is only four times the U.S. maximum exposure limit. Phosgene is water-sensitive (reacting to form corrosive hydrogen chloride gas) and is therefore hazardous to store, ship, and handle. Diphosgene (trichloromethyl chloroformate) and triphosgene (bis(trichloromethyl)carbonate) are alternatives to phosgene with higher boiling points. While these compounds can be used to perform similar reactions with fewer handling difficulties, they still possess toxicities similar to phosgene. Moreover, handling of phosgene and tri-phosgene is labor intensive, and reactions must be performed at −78° C. with exhaustive work ups. Reaction of an active-hydrogen compound such as an alcohol, amine, or thiol with one of these phosgene-based reagents produces hydrochloric acid. The highly acidic hydrochloric acid can decompose acid-sensitive moieties in the starting material. Steps must be taken to scavenge this corrosive gas. These concerns add substantial cost to compounds produced with this reagent.
Known alternatives to phosgene include activated carbonyl compounds such as p-nitrophenyl chloroformate, trichloromethyl chloroformate, carbonyl diimidazole, bis(o- or p-nitrophenyl)carbonate, and bis(2,4-nitrophenyl)carbonate. However, these reagents often suffer from unwanted side reactions, difficult work ups, or lower reactivity.
Thus, a need exists for phosgene substitutes that are environmentally less toxic, less costly, and more compatible with the general goals of “green” chemistry.